improve support for argreduction comparisons

[keewis]

The arg reduction methods in Variable return behave differently depending on the dims parameter:

• for dim=... or dim=None: return a dict containing the indices for all dims
• for dim as a list: return a dict containing the indices for the given dims
• for dim as a hashable: return a single int index for the raveled array

We thus need to construct the proper expected value to be able to check the result.